The main aim of this proposal is to measure associations between physical activity and obesity in a large, population based cohort of children recruited before birth and followed up to the present day. The cohort- the Avon Longitudinal Study of Parents and Children (ALSPAC) - has the global aim of identifying the genetic and environmental determinants of growth and health in children. Over 14,000 pregnant women were recruited to the study during 1991-92 and their offspring are now 10 years old. We propose to measure levels and patterns of physical activity on all children at ages 11 and 13 and to assess relationships with measures of obesity. Physical activity will be measured objectively for 5 days. The data, together with other data collected during pregnancy, infancy and childhood will be used to investigate relationships between modes, intensities, quantities and patterns of physical activity and the development obesity at this critical stage of life. The strengths of this prospective study are its size, high retention rates and the extensive set of whole-of-life measures already taken on the children. With over 10,000 children expected to attend annually for health checks throughout adolescence, the study will be adequately powered to quantify associations between physical activity and obesity. The study of physical activity in children has to date been seriously restricted because of high levels of measurement error inherent in all self-report measures of physical activity. In this study, physical activity will be measured using a single-plane accelerometer (CSA model 7164) - a small, lightweight electronic motion sensor attached to the child's belt for a period of 5 days. This instrument gives the capacity to to characterize children's physical activity, and inactivity, more accurately than ever before. Comprehensive measurements of diet and body composition (principally Dual-emission X-ray Absorptiometry - DXA) will also be taken at ages 11 and 13 under separate funding. The proposed study has the capacity to make important contributions to our understanding of how levels and patterns of activity and inactivity during childhood impacts upon the development of overweight and obesity. These data will provide important insights to how the obesity epidemic can be tackled during childhood.